The present invention relates to a press roll intended to cooperate with a counter roll to form a press with a long nip for paper or board machines.
Presses with a long nip for paper or board machines are known for instance through U.S. Pat. No. 4,563,245, U.S. Pat. No. 4,584,059, U.S. Pat. No. 4,625,376, U.S. Pat. No. 4,707,222, DE 3 607 941 and DE 3 805 350. The press roll of such presses comprises two end walls, a tubular, flexible, liquid-impervious jacket secured at its edge portions by means of an attachment means to peripheral portions of the end walls; stationary support members for supporting the end walls, said end walls comprising bearing means for permitting rotation of the jacket and the end walls in relation to the stationary support members about an axis of rotation; and a press shoe intended together with the counter roll to form a pressing zone with said long nip, the jacket upon rotation being moved through the pressing zone in sliding contact with the press shoe. Within the pressing zone in said known presses, the jacket is forced inward by means of the counter roll to said sliding contact with the press shoe. The part of the jacket pressed against the press shoe will then lie in an axial line located radially inside a line extending between the peripheral end wall portions forming the attachment locations for the edge portions of the jacket, the radial distance between these lines being between 20 and 40 mm. High local axial strains occur in the jacket due to this pressing, when the jacket passes through the pressing zone. These local strains consequently cease immediately after the jacket has left the pressing zone. The strain thus occurring repeatedly in the jacket causes not only fatigue in the material, but also causes considerable tensions at the attachment locations on the end walls and on the end wall bearings. The service life of the jacket becomes too short and both jacket and end wall bearings must be replaced regularly with consequential stoppages and loss of production. To secure the jacket to the end walls attachment means are used which include holes and/or recesses in the jacket itself for inserting bolts which by means of a clamping ring clamp the jacket against the end wall. Local strains occur at the holes and recesses which may cause the jacket to be damaged and the tension will be uneven around the circumference. Furthermore, the jacket may come loose at one or more points because the clamping ring and bolts provide insufficient clamping force to retain the jacket in the desired taut state. The use of holes and recesses in the edge portions of the jacket results in increased costs for manufacturing the jacket in accordance with carefully specified dimensions for each axial size of press roll.
DE Al 3 338 487 describes special connection means disposed at the edges of the jacket and the peripheral end wall portions so that the edge portions of the jacket are axially movable in relation to the end walls. The connection means include engagement means formed directly at the edge portions of the jacket, or separate axially movable engagement elements which can bend or roll and which connect the edge portions of the jacket to the end walls via a fixed securing member, or flexible sealing elements which connect the edge portion of the jacket to the end walls via an axially movable securing member, or a combination of the two first mentioned embodiments. The use of engagement means formed in the edge portions of the jacket results in increased costs for manufacturing the jacket in accordance with carefully specified dimensions for every axial size of press roll. The use of axially movable engagement elements, which can bend or roll, results in the risk of the edge portions of the jacket being pulled out of its engagement position after repeated axial movements of the edge portions to and fro. Furthermore, all embodiments of connection means are relatively complicated to manufacture and fit.